This invention relates to a method for preparing aqueous solutions containing high concentrations of solid oxidizers and a feeder for performing the same.
Oxidizers are frequently used in water treatment applications including cooling waters, pools, spas, pulp manufacturing and papermaking and other applications to bleach pulp and other materials, destroy or inhibit the growth of microorganisms, inhibit or reduce slime deposits, and control odors. Many oxidizers are supplied in solid form as these provide maximum storage stability and high active concentrations facilitating material shipment and transfer. In many water treatment applications, including papermaking, oxidizers are applied as solutions. The solid oxidizers are, thus, dissolved in solution prior to application, such as, for example, with a pressurized solid dissolution feeder.
A typical pressurized solid dissolution feeder 100 is shown in FIG. 1. The feeder 100 includes a tank 106, which contains a bed of solid oxidizer 110, with inlet and outlet pipes 102 and 104, respectively. In operation, water is pumped through the inlet pipe 102 via an inlet pump 108. As the water flows through the tank 106, which is generally at pressures of up to 10 atm gauge, some of the solid oxidizer dissolves in the water and flows out of the tank through the outlet pipe 104. Most such tanks have a volume less than or equal to 270 gallons, since larger tanks which can withstand pressures greater than 1 atm gauge are prohibitively expensive.
It is, however, difficult to produce a continuous stream of solution having a constant concentration of oxidizer with standard solid dissolution feeders. The resulting concentration of oxidizer is dependent on the water flow rate through the tank, the water temperature, and the bed volume (also known as the dissolution bed volume) in the feeder. Since the dissolution bed volume is generally small and decreasing during operation, the feeder must frequently be adjusted to maintain a constant concentration of solid oxidizer in the resulting solution, i.e., to provide a constant delivery rate of solid oxidizer. Due to these difficulties, the concentration of oxidizer in solutions prepared from a solid dissolution feeder typically decrease over a given run requiring frequent increases of water flow, thereby reducing water utilization efficiency and increasing the required level of attention.
Water utilization is an increasing concern. Trends are toward increased closure and increased cycles of concentration in recirculating water loops of high water utilization industries, such as pulp and paper production and cooling. In order to minimize water usage during the application of a solid oxidizer, it is preferable to have a feeder which yields a solution saturated or nearly saturated with the applied product. Furthermore, it is desirable to have a feeder which produces solutions having a constant or near constant concentration of oxidizer over extended periods of time, thus minimizing required adjustments and oversight. Additionally, large volume feeders are desirable as refill frequency is reduced and bulk packaging (such as supersacks) can be directly utilized.
A continuous method is provided for dissolving a solid material, such as a solid oxidizer, into an aqueous solution which yields a product stream of the aqueous solution having a more constant concentration of the solid material than is achieved with standard dissolution feeders. For purposes of simplicity only, the solid material will be discussed herein in terms of being a solid oxidizer; however, it will be understood that the present invention is not limited to the use of solid oxidizers as the solid material and a number of other solid materials that are capable of dissolving in the aqueous solution can be used in the present feeder. In other words, the concentration of solid oxidizer in the product stream obtained from the present feeder decreases less over time in comparison to product streams obtained from traditional feeders. The method of the present invention includes (a) disposing a bed of solid oxidizer into a container; (b) introducing an aqueous solution below the top surface of the bed; and (c) removing solution from the top surface of the bed while maintaining a constant or near constant combined volume of aqueous solution and solid oxidizer (hereinafter referred to as the xe2x80x9cCombined Volumexe2x80x9d) in the container. Preferably, step (c) is performed at about atmospheric pressure. The volume of the bed is preferably maintained at a volume that is at least sufficient to yield a product stream having a concentration of solid oxidizer which is at least 30% by weight of the saturation concentration (i.e., the concentration of the solid oxidizer in the aqueous solution when saturated with the solid oxidizer).
This method can be achieved with a feeder of the present invention. The feeder includes a tank for containing a bed of solid oxidizer, a disperser for introducing the aqueous solution into the tank and a controller. The tank has a bottom, an inlet, and an outlet. The disperser is in fluid communication with the inlet of the tank and is below the top surface of the bed. Preferably, the disperser is at or near the bottom of the tank. As the aqueous solution is dispersed from the disperser, it flows through the bed of solid oxidizer until it rises above the bed as an aqueous solution product. The aqueous solution product above the bed typically contains a high concentration of the solid oxidizer (e.g., having a concentration of at least 30% or at least greater than 60% of the saturation concentration) and is removed from the tank through the outlet. The Combined Volume in the tank is maintained at or near a constant volume over time by the controller. The controller refills the tank with water to maintain the Combined Volume. Unlike prior solid dissolution feeders, which require high pressure vessels to handle the typically high unregulated inlet water line pressure which is necessary to obtain aqueous solution flow rates required to provide desired product application rates at the typically low product concentrations (where flow ratexc3x97concentration=application rate), the present feeder can produce equivalent product application rates at atmospheric pressure at low aqueous solution flow rates. Furthermore, the feeder of the present invention can handle large bed volumes of solid oxidizer. The bed volume in prior dissolution feeders was limited since a pressurized tank was needed. Additionally, the feeder of the present invention can optimize aqueous solution usage by maximizing oxidizer concentrations in the resulting solution. As a result, the feeder of the present invention can be produced and operated at a fraction of the cost of a traditional pressurized feeder.
Another embodiment is a method of dissolving a solid oxidizer into an aqueous solution by dissolving the solid oxidizer into the aqueous solution with the feeder of the present invention.
Yet another embodiment is a method for calibrating the feeder. The method includes the steps of first selecting a flow rate of the aqueous solution discharged from the tank. Second, a solubility curve for varying bed volumes (or apparent residence times) is prepared at the selected flow rate. Typically, as the bed volume (or apparent residence time) decreases, so does the solid oxidizer concentration in the product stream. Third, an initial bed volume for the solid oxidizer is selected based on the solubility curve for the feeder. Preferably, the initial bed volume is sufficient to yield a product stream having a solid oxidizer concentration of at least about 50, 60, 70, 80, 90, 95, 96, 97, 98, or 99% of the saturation concentration. Fourth, a minimum (or final) bed volume is selected based on the solubility curve for the feeder. The minimum bed volume is generally selected based on acceptable tolerance limits for the variation of the solid oxidizer concentration in the product stream. Generally, the minimum bed volume is that which produces a solution concentration of about 30% of the saturation concentration. During operation, the bed volume is initially set at the initial volume and then over time as the solid oxidizer dissolves into the aqueous solution, the observed bed volume decreases. Preferably, once the observed bed volume reaches the minimum bed volume, an action is taken. For example, more solid oxidizer is added to the tank or the feeder process is stopped.
The feeder of the present invention is particularly useful in papermaking, industrial cooling and other high volume applications.